I had problems with the 3.0 OS on my 2G and with battery life as well. There aren't many things to try, but here's a list:
1) If you set up as a restore from backup, wipe it and set up as a new phone. This fixes 90% of people with battery issues from 2G. Yes, you lose your call logs, SMS history, etc. You can back those up if you really want to, just save your most recent iPhone backup from the iTunes directory before wiping the device.
2) Push notifications. If you've hacked / pushfixed / etc., this can be a problem. Make sure you've run pushfix if it's applicable to your situation.
3) Apps. Once you restore the device, don't do a sync on it. Let it run for one full day before putting any apps / videos / music on it. Sync your contacts if need be, but leave Apps and Media alone. Use this day as a solid test of the battery life. If it sucks now, it's a hardware problem in 99.99999% of cases.
4) If your battery life was fine on step three, start by syncing your media and videos, but leave apps out of it. Do another full-day test.
5) If your battery life was fine on step four, install apps one at a time, or in groups, so that if your battery life drops through the floor you can identify either which app or which group of apps is causing it.
In my case, Facebook was the culprit -- even though push notifications were disabled for my device itself, because Facebook was set to check for notifications, it apparently was doing that. Removing Facebook fixed it, then reinstalling and turning off all notification checks from Settings.app in the General Settings location permanently fixed it.
Aside from what's listed above, there really isn't much other than what Oletheros suggested...